Manage AgentUpdates more sanely:

- The existing event to scene has been split into 2: OnAgentUpdate and OnAgentCameraUpdate, to better reflect the two types of updates that the viewer sends. We can run one without the other, which is what happens when the avie is still but the user is camming around
- Added thresholds (as opposed to equality) to determine whether the update is significant or not. I thin these thresholds are ok, but we can play with them later
- Ignore updates of HeadRotation, which were problematic and aren't being used up stream

OpenSim/Framework/IClientAPI.cs

@@ -825,6 +825,8 @@ namespace OpenSim.Framework
         /// </remarks>
         event UpdateAgent OnAgentUpdate;
 
+        event UpdateAgent OnAgentCameraUpdate;
+
         event AgentRequestSit OnAgentRequestSit;
         event AgentSit OnAgentSit;
         event AvatarPickerRequest OnAvatarPickerRequest;